1. Field of the Invention
The present invention generally relates to a signal processing apparatus, a signal processing method therefor, an information recording apparatus and an information reproduction apparatus, and more particularly to a signal processing apparatus, a signal processing method therefor, an information recording apparatus and an information reproduction apparatus in which a partial response method is used.
2. Description of the Related Art
Recently, a recording density of magnetic disks is increasing rapidly. This is because both high-sensitive MR (magneto-resistive effect) heads and a PR4ML (a partial response class 4 and a maximum likelihood detection) method are introduced into the magnetic disk drives.
FIG. 1 shows a block diagram of a signal processing circuit of one example according to the prior art.
A signal processing circuit 1 comprises a pre-coder 2, an NRZI (non-return-to-zero interleave) recording system 3, a differential detection block 4, a magnetic reproduction system 5, an equalizer 6, a level detection block 7 and a maximum likelihood detection block 8.
An 8/9-conversion RLL (Run Length Limited) code is supplied to the pre-coder 2 as an input code. The input code is pre-coded by the pre-coder 2 according to 1/(1+D), where D indicates a one-bit delay.
The pre-coded code is converted into an NRZI-code by the NRZI recording system 3 and the NRZI-code is recorded on a magnetic disk with a step-shaped recording current. A recorded step-shaped signal is differentially detected by a read head in the differential detection block 4. Thus, an impulse waveform of the recorded signal is detected by the differential detection block 4.
The output impulse waveform from the differential detection block 4 is amplified and noise components are removed from the impulse waveform by the magnetic reproduction system 5.
Then, an operation is performed on an output signal from the magnetic reproduction system 5 by the equalizer 6 according to (1+D), where D indicates a one-bit delay.
The level detection block 7 generates a ternary-level signal from the output signal of the equalizer. The maximum likelihood detection block 8 corrects errors in the ternary-level signal.
FIG. 1 shows a block diagram of a PR4ML method. An operation is performed on a bit sequence of the input code according to 1/(1+D)xc3x97(1+D), where D indicates a one-bit delay, according to the partial response method. The noise 25 components are also reduced by the term (1+D) of the equalizer 6. Next, the ternary-level signal is generated from an output signal of the equalizer 6 by the level detection block 7. Then, the maximum likelihood detection block 8 corrects errors caused by the noise components remaining in the ternary-level signal with a Viterbi algorithm.
FIG. 2 shows a transfer characteristic of the term (1+D) used in the PR4ML method. A high-frequency noise component is reduced because the transfer characteristic of (1+D) is a low-pass filter. Therefore, a signal-to-noise ratio of the output signal from the equalizer 6 is increased.
As mentioned above, the 8/9-conversion RLL code is employed as the input code in the signal processing circuit 1 shown in FIG. 1 together with the PR4ML method.
However, a recording frequency for recording the 8/9-conversion RLL code used in this signal processing circuit 1 with the PR4ML method is higher than that of a 1/7-conversion RLL code under the condition that the same recording density is achieved by both codes. As the recording frequency increases, the distortion of a recording waveform also increases. This distortion causes a bit shift (NLTS, Non Linear Transition Shift) of reproduced data. Therefore, this causes a problem that an error rate of the reproduced data is increased.
On the other hand, a 1/7-conversion RLL code may be used in a system together with a peak detection method. Therefore, it is possible to lower the recording frequency of the system using the 1/7-conversion RLL code together with the peak detection method below that of the system using the 8/9-conversion RLL code together with the PR4ML method. However, it is very hard to employ a maximum likelihood decoding circuit because whether a signal exists in a time window or not is only detected by the system using the peak detection method.
It is a general object of the present invention to provide a signal processing apparatus and a signal processing method therefor, an information recording apparatus and an information reproduction apparatus in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a signal processing apparatus, a signal processing method therefor, an information recording apparatus and an information reproduction apparatus in which a low recording frequency, a reduced distortion of a waveform of a recording current and a reduced NLTS (Non Linear Transition Shift) are achieved and an error rate of a reproduced signal is reduced by a maximum likelihood decoding method.
The above objects of the present invention are achieved by a signal processing apparatus in which a signal is processed using a partial response method. The apparatus comprises a signal processing unit which performs an operation (1xe2x88x92D) on each bit of an input code, where D indicates a delay of one bit; an equalizing unit which equalizes a bit sequence processed by the signal processing unit to a Nyquist characteristic; and a maximum likelihood detection unit which detects a maximum likelihood bit sequence from the bit sequence equalized by the equalizing unit. The input code of the signal processing unit is a 1/7-conversion RLL code of input data.
According to the invention, an error rate of a signal reproduced by the signal processing apparatus can be reduced by a maximum likelihood decoding method using a partial response method.
Furthermore, according to the invention, a low recording frequency, a reduced distortion of a waveform of a recording current and a reduced NLTS are achieved by using the 1/7-conversion RLL code. This also leads to a reduction of an error rate of the reproduced signal.